Mechanisms responsible for nonsusceptibility to infection with Schistosoma mansoni in Biomphalaria glabrata snails are not known. Among the hypotheses proposed are physiological unsuitability and immunological resistance, the latter involving rapid, hemocyte-mediated destruction of the primary sporocyst following recognition of the parasite as nonself (e,g., by plasma or cell surface lectins) and/or failure to recognize the parasite as self. In B. obstructa, a North American species totally refractory to infection with S. mansoni, most sporocysts are not destroyed by a hemocytic response, but instead degenerate, and sporocyst survival is increased in snails infected with Echinostoma paraensei. This proposal seeks to (1) ascertain the fate of S. mansoni in echinostome-infected B. obstructa; (2) ascertain the fate of S. mansoni in snails naturally-infected with Ribeiroia ondatrae or a strigeid; (3) determine whether enhanced resistance occurs against E. paraensei; and (4) study the mechanism of nonsusceptibility to S. mansoni.